Steel or cast iron pipes are being increasingly replaced with equivalents made of plastic.
Polyolefins, especially polyethylenes, are very widely used thermoplastics as they exhibit good mechanical properties, they can be easily converted and allow pipes to be welded together easily. Polyolefins are widely used for the manufacture of pipes for transporting water or town gas. When the gas is under a high pressure (>10 bar or higher), it is necessary for the polyolefin to mechanically withstand the stresses exerted by the pressurized gas.
In addition, the polyolefin may be exposed to an aggressive chemical environment. For example, in the case of water transport, the water may contain additives or aggressive chemicals (for example ozone, and chlorinated derivatives, used for the purification of water such as bleach, which are oxidizing, especially when hot). These additives or chemicals may damage the polyolefin over the course of time, especially when the water transported is at a high temperature (this is the case in heating circuits or in water systems for which the water is heated to a high temperature in order to eliminate germs, bacteria or micro organisms). One problem that the invention aims to solve is therefore to develop a chemically resistant pipe.
Another problem that the invention aims to solve is that the pipe must have barrier properties. The term “barrier” is understood to mean the fact that the pipe reduces the rate of migration into the transported fluid of contaminants present in the external environment or else contaminants (such as antioxidants or polymerization residues) present in the polyolefin. The term “barrier” also means the fact that the pipe reduces the rate of migration of oxygen or of additives present in the transported fluid into the polyolefin layer.
It is also necessary for the tube to have good mechanical properties, in particular good impact strength, and for the layers to adhere well to one another (no delamination).
The Applicant has developed a multilayer pipe that solves the stated problems. This pipe has in particular good chemical resistance to the transported fluid and the abovementioned barrier properties.
Document EP 1484346 published on 8 Dec. 2004 describes multilayer structures that include a radiation-grafted fluoropolymer. The structures may be in the form of bottles, tanks, containers or hoses. The structure of the multilayer pipe according to the invention does not appear in this document.
Document EP 1541343 published on 8 Jun. 2005 describes a multilayer structure based on a fluoropolymer modified by radiation grafting in order to store or transport chemicals. In this application, the term “chemicals” should be understood to mean products that are corrosive or dangerous, or else products whose purity has to be maintained. The structure of the multilayer pipe according to the invention does not appear in this document.
Document U.S. Pat. No. 6,016,849 published on 25 Jul. 1996 describes a plastic pipe in which the adhesion between the internal layer and the external protective layer is between 0.2 and 0.5 N/mm. There is no mention of a fluoropolymer modified by radiation grafting.
Documents US 2004/0206413 and WO 2005/070671 describe a multilayer pipe comprising a metal casing. There is no mention of a fluoropolymer modified by radiation grafting.